Many systems in an engine resonate and come into resonance at different RPMs.  When something comes into resonance, it can be constructive and make more power or make less power if it is destructive resonance.  The double hump power curve shown most often indicates the ports are optimized for one RPM and the pipe is tuned to be optimum at another RPM.  JH

I've asked this question about the dip with your pipe in the Zilla shootout. I finally have the answer. Thanks